The present invention relates to a mine roof truss bolt system, and particularly to a clip for use in such system.
Truss-bolt-type systems are increasingly being used to provide additional support for mine roofs over that provided by conventionally used systems consisting of patterns of spaced vertical bolts. Truss-bolt systems are mostly used in wide entries to a long wall panel or in cross entries where the span between walls or pillars is great. The truss-bolt systems generally include two opposed anchor bolts inserted in angled holes drilled in the roof. The holes are drilled at an angle and depth so that the anchors extend upwardly to locations over the mine pillars adjoining the passage. At the lower end of each anchor, clips usually are provided to receive the anchor and a transverse reinforcing member extending laterally between the clips just below the roof face. The anchors and reinforcing member are tensioned by various devices so as to apply an upward and inward force for additional support of the roof.
There are various truss-bolt systems known in the art. U.S. Pat. No. 3,505,824, White discloses a system in which the rods extend angularly into the roof and are bent inwardly toward the center of the mine opening in overlapped relationship. The personnel installing this system must bend the rods manually after they are secured in the holes drilled in the roof. After bending, the rods are secured together and tensioned in a central area of the mine passage. Thus, not only is installation difficult and time-consuming, but it also requires that the personnel stand in the center of the passage where material from the roof is most likely to fall on them. This sytem is also described in an article published in COAL MINING AND PROCESSING, June 1969, pages 64, 65 and 83, and a paper published in the SOCIETY OF MINING ENGINEERS, AIME, June 1970, volume 249, pages 109, 110.
Another truss-bolt system shown in U.S. Pat. No. 3,427,811, White also requires that the operators stand in the center of the passage in order to apply tension to the reinforcing member. The reinforcing member in this latter system consists of two rods secured to clips at their outer ends and joined in the central passage area by a turnbuckle. U.S. Pat. No. 3,509,726, White discloses a truss-bolt system composed of two anchor members extending angularly into the roof and a reinforcing member joined at its outer ends by straps, each of which is secured in a wedge device. The opposite end of each strap is mounted in a special collar installed on the lower ends of the anchor members. The collars automatically apply tension to the reinforcing member when tension is applied to the anchor members. Installation of this system is difficult because of the relatively large number of parts. It also suffers from the disadvantage that blocks must be used between the reinforcing member and the roof reducing the amount of head clearance in the passage. Finally, U.S. Pat. No. 4,274,762, Johnson shows a truss-bolt system which requires tensioning of the reinforcing member prior to the drilling of angled holes for the anchor members. This latter system requires special equipment for installation. All of the above systems suffer from the disadvantage that the tension applied cannot be readily measured. Thus, it is often found after installation that the reinforcing member is loose and does not serve to create an upward force on the anchor members. This defeats the purpose of truss-bolting and may be hazardous in that no additional roof support is actually provided.
Another deficiency in each of these systems is that the application of force on the reinforcing member produces a force moment acting about a horizontal axis at the location where the anchor members are tensioned. This force moment tends to reduce the actual upward force applied to the roof and also creates stresses tending to weaken the clip itself.
It is therefore a primary object of this invention to provide a mine roof truss-bolt system which overcomes the disadvantages of the prior art discussed above.
It is another object of the invention to provide a mine roof truss-bolt system and clips for use in such system designed to eliminate force moments acting on said clips about horizontal axes normal to the axes of the anchor members and reinforcing member at the locations at which said members are connected to the clips.
It is still another object of the invention to provide a mine roof truss-bolt system which is easier and safer to install than prior systems.
It is a further object of the invention to provide a mine roof truss-bolt system which may be installed in separate stages.
It is still a further object of the invention to provide a mine roof truss-bolt system in which the degree of tension applied to the anchor and reinforcing members may be accurately measured so as to assure that proper support for the roof is provided.